1. Field of the Invention
The present invention relates to an optical scanning device that uses a vibrating mirror as a light deflector and an image forming apparatus that includes the optical scanning device.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2007-155984 discloses an optical scanning device that uses a vibrating mirror as a light deflector and an image forming apparatus that includes the optical scanning device. The optical scanning device scans a target surface with a light beam in the main-scanning direction. The optical scanning device includes a light source that emits a light beam, a movable member that has a deflection mirror surface, and a vibrating unit that vibrates to drive the movable member around a drive axis in a direction substantially perpendicular to the main-scanning direction due to an expansion and contraction movement of at least one piezoelectric actuator. The optical scanning device further includes a deflecting unit that deflects the light beam emitted by the light source by using the deflection mirror surface, thereby causing the target surface to be scanned with the deflected light beam in the main-scanning direction. In addition, the optical scanning device includes a mirror driving unit that vibrates the movable member by applying a mirror drive signal to the piezoelectric actuators, wherein the mirror drive signal has a periodically changing voltage. The mirror driving unit applies a signal generated by adding an offset voltage to the mirror drive signal to all or some of the piezoelectric actuators, thereby adjusting the center of the vibration of the deflection mirror surface. The offset voltage is used to displace the center of the vibration of the deflection mirror surface, and the amount of displacement of the center of the vibration is determined based on the offset voltage.
According to Japanese Patent Application Laid-open No. 2007-155984, the center of the vibration of the deflection mirror surface is adjusted by applying the offset voltage. The size of a drive circuit for applying the offset voltage to a piezoelectric element needs to be large to adjust the center of the vibration of the deflection mirror surface with high accuracy. Furthermore, although the center of the vibration of the deflection mirror surface is adjusted in a forcible manner, it can be only adjusted within a limited range. Moreover, because the optical scanning device is designed to allow for large vibration amplitude of a torsion bar, the piezoelectric actuator needs to have high drive capability; therefore the piezoelectric actuator needs to be large and high costs are incurred.
In an image forming apparatus such as a color image forming apparatus that uses a polygon scanner as a light deflector, the polygon scanner needs to be rotated at a speed higher than 25,000 revolutions per minute (rpm) with high accuracy in order to achieve high-speed printing and high image quality. However, there are problems with the above apparatus. Because of the high image quality achieved by reducing the diameter of a laser beam, a polygon mirror used in the polygon scanner has an incircle with a relatively large radius and relatively long length in the main-scanning direction; therefore, the load on a polygon motor that drives the polygon mirror is likely to be heavy. Due to the heavy load on the polygon motor, electric power consumption of the polygon scanner is increased and heat generated by the polygon scanner adversely affects optical elements such as a scanning lens. The scanning lens is affected by the heat most severely because the scanning lens is located closest to the polygon scanner. The heat of the polygon scanner is transmitted to the scanning lens via an optical housing or due to radiation, resulting in an increase in temperature.
The temperature increase of the scanning lens is not uniform due to its distance from the polygon scanner, i.e., the heat generation source in this case, different coefficients of thermal expansion depending on the material of each component, and influence of air currents. Particularly, if the scanning lens has a temperature distribution in the longitudinal direction, i.e., in the main-scanning direction, the form accuracy of the scanning lens is decreased and the refractive index of the scanning lens is altered. As a result, a spot position of the laser beam on the target surface varies and the quality of formed images deteriorates. These problems become noticeable if the scanning lens is made of plastic having a high coefficient of thermal expansion. Furthermore, in the color image forming apparatus, because target surfaces are individually scanned by laser beams corresponding to four colors, i.e., yellow, magenta, cyan, and black, temperature deviation occurs between optical scanning devices that correspond to the four colors. The temperature deviation causes incorrect relative positions of beam spots that correspond to the four colors, which results in color shift of images.
The increase in temperature due to the heat generated by the heavily loaded polygon mirror causes a slight displacement of components included in a rotating body, especially the polygon mirror that constitutes a large fraction of the rotating body's total mass. The slight displacement alters the balance of the rotating body, which results in the occurrence of vibration. If components included in the rotating body, such as the polygon mirror, a rotor magnet of the polygon motor, a flange to which the rotor magnet is attached, or a shaft, have different coefficients of thermal expansion, or even if these components have the same coefficient of thermal expansion, the components included in the rotating body are slightly displaced (the balance of the rotating body is altered) due to the high speed rotation of the polygon mirror at the increased temperature whereby the vibration is increased. To prevent such a problem, component tolerances and methods for fixing the components need to be strictly controlled and monitored. The vibration is transmitted to an optical element included in the optical scanning device. This optical element can be a mirror that changes an optical path. The vibration is amplified by the optical element whereby banding (density variation in the sub-scanning direction) occurs, and as a result, image deterioration and noise are caused.